What Is the Tide? 

Source of pictures: pixabay.com

The warm sunlight cast down, giving a lethargic feeling. The tiny feet left footprints on the beach one after another. The seawater sometimes rushed up to that pair of feet and brought away sand remaining within toes. Mackle was having a good time at the beach, leaving a happy figure. Thanks to parents' hard-earned vocation, he could come here to enjoy a rare journey. Due to a rare chance for travelling, his parents decided to take him to the sea because he was yearning for it a long time ago.

However, Mackle, a small "scientist", already got a plan in his mind. He intended to "do research". What would he research on? Mackle had a new theme in his science class. The teacher divided the students into groups. Each group should figure out the cause of a natural phenomenon and do a report to the whole class. This should not be difficult for Mackle because he knew a lot. However, he wanted to do something different this time. He wanted to go into nature, go on field trips and record details to do a real "exploration" like a naturalist. This holiday gave him such a wonderful opportunity. The sea was what he wanted to explore for a long time. He was especially interested in the rise and fall of the tides, which was one of the most mysterious natural forces.

Mackle recoded the time while his ankles were slowly overwhelmed by the flow of the sea. Looking at the furthest part of the blue sea, he was lost in thought: "What is the tide?"

Unexpectedly, the tide is actually controlled by "the sky"! This "sky" refers to the moon and the sun. In fact, the tides are aroused by the sun and the moon, especially the moon. How do these celestial bodies in the sky affect the seawater on earth?

Before talking about this question, we have to get straight two concepts. One is "inertia", another is "universal gravitation". "Inertia" is the force to remain a moving object's original state of motion when the motion state has changed. For example, when the car makes a turn, we always tilt to the opposite direction and feel like we are about to be thrown out. This is caused by the inertia. "Universal gravitation" refers to the mutual force between any two objects.

Let's move onto the tide. As we all know, the moon is moving around the earth while the earth is spinning. The tide is the result of the universal gravitation of the moon (and the sun, whereas the moon plays a greater role) on the earth and the inertia generated by the rotational motion of the earth. A forced called as "tidal force" is formed by the combination of the universal gravitation and the inertia. It always pulls the seawater within an area to the direction of moon. People on the ground regard it as the rise and fall of the sea. The tide occurs at a specific time twice a day in the daytime and at night respectively.

Do you think Mackle has figured out this secret of nature?